Osteoporosis is a widely prevalent contributor to frailty, increased morbidity, and mortality in the elderly and results from an imbalance between bone resorption and bone formation. Sirtuins have been implicated in metabolism, longevity, and healthspan. Resveratrol (RSV) is a sirtuin activating compound that enhances bone mineral density (BMD) in mice, and sirtuin knockout mice exhibit increased osteoclastogenesis. SRT2183 and SRT1720 are synthetic resveratrol mimetics (SyRMs) that were designed to specifically activate sirtuin1 (Sirt1), but an increasing number of reports suggest that RSV and SyRMs act on targets other than Sirt1. Our preliminary data demonstrate that: 1) SyRMs inhibit c-Src phosphorylation, actin ring formation, osteoclast formation, and bone resorption, but RSV does not; 2) SyRMs also disrupt actin rings and inhibit bone resorption in mature osteoclasts, but RSV does not; 3) knockout of Sirt1 expression impairs bone quality in vivo and enhances osteoclastogenesis ex vivo, but SyRMs still inhibit actin ring and osteoclast formation in Sirt1 knockout cells; 4) RSV treatment of young mice increases osteoblastogenesis and decreases osteoclastogenesis; and 5) RSV treatment of very old mice markedly improves bone mass and bone quality. Hypothesis and Specific Aims: We hypothesize that SyRMs inhibit critical pathways of osteoclast formation and function involving c-Src phosphorylation, Rho GTPase activation, and NFATc1 expression. Intriguingly, despite lack of effect in vitro, RSV exerts pronounced benefit in vivo, suggesting that the predominant effects are on osteoblasts. We propose that in vivo treatment with RSV and SyRMs will both restore the youthful balance between osteoclastic bone resorption and osteoblastic bone formation and enhancing bone mineral density and bone quality, but possibly via different mechanisms. We also hypothesize that the SyRMs can exert beneficial effects upon bone independent of Sirt1. 1. Determine the mechanisms whereby RSV, RSV mimetics, and Sirt1 regulate osteoclast and osteoblast formation. The results of this aim will answer the critical questions of whether or not SyRMs inhibit osteoclastogenesis by inhibiting c-Src phosphorylation, Rho GTPase activation, and NFATc1 expression and whether the effects of SyRMs depend upon Sirt1. 2. Determine the skeletal impacts of RSV, RSV mimetics, and Sirt1 in vivo. The results of this aim will answer the critical questions of whether or not in vivo RSV and SyRMs prevent age-related and/or ovariectomy-induced bone loss, whether RSV and SyRMs exert either anabolic and anti-resorptive effects on bone or both, and whether the impacts of RSV and SyRMs depend upon Sirt1 gene expression. There is a pressing need for targeted agents to improve the quality of bone and to reduce the adverse consequences of osteoporosis. Our proposed studies will permit us to understand better the mechanism(s) underlying the impacts of RSV and SyRMs and their relationship to Sirt1 expression. Our work will also have significant clinical impact given the widespread and deleterious burden of osteoporosis in the elderly and can be applied to the utilization of RSV and RSV mimetics in the clinic.